1. Field of the Invention
The present invention relates to a resin-sealed semiconductor device such as a diode, a thyristor, a transistor module, etc.
1. Description of Related Art
FIGS. 1 to 3 show a conventional resin-sealed semiconductor device. More specifically, FIG. 1 is a cross-sectional view of a conventional resin-sealed semiconductor device. FIG. 2 is a perspective view showing the conventional resin-sealed semiconductor device shown in FIG. 1. FIG. 3 is an enlarged partial, cross-sectional view showing the conventional resin-sealed semiconductor device shown in FIG. 1. In FIG. 1, 20 is a metal heat sink, 22 is a casing made of a resin and connected to the heat sink 20 with an adhesive. On the heat sink 20, there are provided two insulators 24, each of which has metalized surfaces, and conductor layers or patterns 26 formed on the metalized insulators, respectively, so that there are two "islands" each consisting of the insulator 24 and the conductor layer 26 superimposed one on another. A semiconductor chip 28 is mounted on each island. That is, the semiconductor chips 28 are mounted, via the insulators 24 and the conductor layers 26, on the heat sink 20 side by side at a certain distance from each other. The chips 28 are welded to the conductor layer 26. An upper electrode 30 is connected to each of the semiconductor chips 28, and an output terminal 31 is connected to the conductor layers 26. The upper electrodes 30 and the output terminal 31 extend outward through holes provided in an upper wall 22A of the casing 22. A sealing resin 32, such as an epoxy resin that contains an aggregate 34 such as quartz particles, fills an inner cavity defined by the casing 22. The aggregate 34 is added to the sealing resin 32 in order to reduce its amount as well as to increase its strength after its hardening.
In a resin-sealed semiconductor device having such a structure as described above, the sealing resin 32 tends to peel off from the heat sink 20 to form a gap therebetween. The gap formation causes various troubles and defects.
More particularly, as shown in FIG. 2, an increase in the temperature of the semiconductor device when it is heated, for example, in a heat cycle test will bring about a displacement of the heat sink 20, which generally has a thickness of about 3 mm, the displacement being in the direction indicated by an arrow P in an X-Y plane defined in an imaginary X-Y-Z space. On the other hand, the sealing resin 32 filled in the cavity in the casing 22 has a coefficient of thermal expansion different from that of the metal heat sink 20, and is displaced in the direction indicated by an arrow Q in the space X-Y-Z. Therefore, there arises a tensile force between the sealing resin 32 and the heat sink 20 due to a relative displacement therebetween. Since the sealing resin 32 contains a multiplicity of particles of the aggregate 34, such as glass particles, some of the particles of the aggregate directly contact the surface of the heat sink 20 to restrict or reduce the effective contact area in which the sealing resin itself contacts the surface of the heat sink 20. As a result, the bond strength between the resin and the heat sink decreases. Due to this as well as to the above-described difference in the coefficient of thermal expansion and relative displacement between the resin and the heat sink 20, peels tend to occur readily on an interface between the heat sink 20 and the sealing resin 32 filled in the cavity or space of the casing 22. In accordance with the peeling between the resin and the heat sink, there also occurs peeling on the surface of a marginal portion 20A of the heat sink 20 on which an end portion 22B (FIG. 3) of the casing 22 bonds.
FIG. 3 illustrates the state in which such a marginal peel has occurred. In FIG. 3, the symbol "g" indicates a gap formed between the heat sink 20 and the casing 22 or the sealing resin 32. In case there is a gap between the heat sink 20 and the end portion of the casing 22, moisture in the air outside penetrates into the sealing resin 32 through the gap g to deteriorate its moisture resistance. In addition, if there is a gap between the heat sink 20 and the sealing resin 32, a large stress is exerted to the semiconductor chips 28 welded to the respective conductor layers 26. As a result, cracks occur in the semiconductor chips. This is a major cause for the deterioration of not only reliability of the device but also good product rate.